A simple compression refrigeration system generally comprises a compressor; a condenser; a refrigerant metering/expansion device, commonly termed an expansion valve or capillary tube; a heat exchanger, commonly termed an evaporator; and a temperature controller. Refrigerant vapor is compressed by the compressor, and in so doing raises the pressure and the temperature of the refrigerant. The vapor is fed into the condenser where the refrigerant releases heat to a cooling medium, resulting in a refrigerant condensate. The condenser cooling medium is generally air blowing over the condenser coils with a fan, or circulating water in a heat exchanger. The condensate then enters the expansion valve where the pressure and temperature of the condensate are reduced to preselected values. The cold refrigerant condensate then enters the evaporator at a reduced pressure. In the evaporator, the condensate absorbs heat from the thermal process and in so doing the condensate vaporizes. The change in temperature between the condensate and the vapor, as well as the latent heat of vaporization absorbed by the refrigerant, cools the thermal process which is in thermal contact with the evaporator. The refrigerant vapor is then recycled back into the compressor, thereby completing the circuit. In refrigeration systems using hermetically or semi-hermetically sealed compressors, the refrigerant vapor returning to the compressor serves to cool the electric motor that drives the compressor. Cooling of the compressor electric motor is an important function.
In response to a thermal process attaining a preselected temperature, a temperature controller shuts off the compressor. The compressor remains deactivated until the thermal process temperature rises above a preselected temperature threshold, at which point the temperature controller reactivates the compressor so as to charge the evaporator with reduced pressure refrigerant condensate. This type of general system has several limitations. The cycling of compressor activity to maintain a desired thermal process temperature results in wide temperature swings and additionally causes undue wear on the compressor. Further, the maximum preselected thermal process temperature is generally limited such that the refrigerant vapor recycled to the compressor is capable of cooling the compressor motor. In general, the refrigerant vapor must be maintained below 70.degree. F. In instances where pressure limiting valves are installed in the recycle line, returning refrigerant vapor temperatures up to about 90.degree. F. are functional.
In an effort to address these limitations, a hot gas bypass valve is installed such that hot gas may selectively be shunted from the inlet of a condenser to the inlet of an evaporator with a temperature controller driving the operation of either the hot gas bypass valve or the compressor. This refrigeration system operates in the same manner as that described above, until the thermal process is in thermal stasis. During this condition the temperature controller opens the hot gas bypass valve and bleeds hot vapor mixed with cold refrigerant condensate into the evaporator to reduce the cooling capacity of the refrigerant entering the evaporator. The activation of the hot gas bypass valve is an alternative to activation cycling of the compressor. The hot gas bypass valve is typically a solenoid valve that is controlled by the temperature controller which is simultaneously monitoring the thermal process heat load. Thus, when cooling is required the hot gas bypass valve is closed and only refrigerant condensate enters the evaporator. When thermal process cooling is not required, the temperature controller activates the hot gas bypass valve and hot vapor enters the evaporator thereby decreasing the instantaneous cooling capacity of the refrigeration system. In other instances, the hot gas bypass valve used in the prior art is a pressure operated valve that senses the recycle line pressure, which is also known as the suction pressure. Upon the thermal process attaining thermal stasis, the suction pressure goes below a preselected set point, thereby activating the hot gas bypass valve so as to allow hot vapor to enter the evaporator until the suction pressure increases to above the set point. Should the hot gas bypass valve have inadequate throughput to shunt all of the hot vapor, then the thermal process continues to cool even though the thermal process is already below the preselected temperature threshold. In response to such a condition, the temperature controller must cycle the compressor activation in order to maintain the thermal process within the bounds of preselected temperature thresholds.
In spite of the improvements associated with the introduction of a hot gas bypass valve and the configuration detailed above, a number, of limitations still persist. For instance, the hot vapor entering the evaporator tends to cause wide temperature fluctuations to the thermal process. In addition, should a hot gas bypass valve be required to shunt a large percentage of the hot vapor, then the refrigerant vapor recycled to the compressor will often be excessively warm to properly cool the compressor motor, thereby causing undue compressor wear and possibly compressor failure. Lastly, the maximum thermal process temperature remains limited in such a system by the requirement that the returning refrigerant vapor temperature be below 70.degree. F. for a high recycle line pressure system, or below 90.degree. F. in those systems where the recycle line is fitted with a pressure limiting valve.
Many laboratory and industrial cooling applications, including the operating of lasers, electron microscopes, ion beam generators and injection molding machines, require a thermal reservoir circulated at stable temperature, flow and heat capacity. Many of these applications require temperatures above 70.degree. F. Since prior art refrigeration systems suffer above this temperature, such applications now use an external source of circulating cooling water functioning as a chiller.
The limitations of existing refrigeration systems is illustrated in regard to plastic injection molding machines, in which molten plastic is injected into a mold. Heat is removed from the molten plastic in order to solidify the molded article by circulating water through the cooling passages of the mold. Typically, a chiller supplies cooling fluid in a temperature range from about 40 to 70.degree. F. A chiller of this design is limited to 70.degree. F. as the maximum temperature due to the refrigeration componentry and design. Many thermoplastic and thermoset materials require cooling temperatures ranging from 100.degree. F. to 200.degree. F. and above in order to obtain the desired properties in the molded article. Currently in order to provide the higher cooling temperatures for some plastic molding applications, a mold temperature controller unit is required. Such a unit is equipped with a pump, heater, valves and controls in order to attain higher temperatures. When cooling is required, a valve system introduces quantities of water from the chiller, whereas heating is provided by the heater. Thus, to operate a conventional "mold temperature controller" an external source of cold water is required.
The instant invention strives to overcome the limitations of conventional cooling systems, as detailed above,